This invention relates to a tension-torsion member in general and a flexstrap for connecting a helicopter rotor blade to a hub in particular.
The rotors of a helicopter rotor system must be permitted three degrees of freedom of movement. Flapping motion or motion in a direction perpendicular to the plane of rotation of the rotor must be permitted. Motion about the rotor blades longitudinal axis, or pitch, is necessary in order to vary the amount of lift generated by a particular blade, and is accordingly necessary in order to control the aircraft. Additionally, lead lag motion or movement of the rotor blade in a direction substantially parallel to the plane of rotation is necessary and must be permitted. One of the primary reasons that flapping and lead lag motion must be permitted is to lower the stresses in the components which secure the rotor blades to the hub.
In conventional helicopters, lead lag, flap and pitch motions are accomplished through hinges which utilize bearings of various types. Due to many problems associated with hinges having bearings such as manufacturing costs, lubrication problems, sizing problems, and difficulty in maintaining manufacturing tolerances, various substitutes were sought.
Among the results of these efforts are the family of rotors known as hingeless rotor systems. Hingeless rotor systems are rotors which permit flapping, lead lag, and pitch motions without the use of conventional hinges. The rotor described in U.S. Pat. No. 3,765,267 issued to Bourquardez and Coffoi issued Oct. 16, 1973, is an example of such a hingeless rotor system. A similar hingeless rotor construction for permitting flap, lead lag, and pitch motions is also taught by Bourquardez and Coffoi in U.S. Pat. No. 3,669,566 issued June 13, 1972.
There are many different constructions for connecting a rotor to a hub without the use of conventional hinges in such a manner so as to permit articulation of the blade with respect to the hub. Some of these constructions have utilized a conventional tension-torsion strap or tie bar. The invention taught by U.S. Pat. No. 3,362,252 to Ditlinger issued Jan. 9, 1968 is an example of such a connecting link. U.S. Pat. No. 3,228,481 issued Jan. 11, 1966 to Eldred shows a tension-torsion strap being utilized in a conventional hinged rotor so as to permit some motion of a rotor blade about its pitch axis. U.S. Pat. No. 3,578,877 issued May 18, 1971 to Mautz also discloses a tension-torsion strap which is used for permitting rotation of a rotor blade about its pitch axis.
There have been various problems associated with hingeless rotor systems which are fully articulated, i.e., which permit flap, lead lag and pitch motions of the rotor blade. In a rotor system it is highly desirable that the degree of force or control loads which are required to rotate the rotor blade about its pitch axis be reduced to a minimum. This has been particularly difficult to achieve in the bearingless flexstrap configurations. In order to reduce the size and weight of the flexstrap of an articulated rotor system, it is necessary that the stresses within the flexstrap be kept to a minimum particularly when the strap is subjected to lead lag and flap motions. Additional, but desirable, features in an articulated flexstrap are control over the placement of the effective lead lag and flap hinge points relative to the hub. As used herein, an effective hinge point is that point about which motion between two members occur.